It has long been an object of chemists to extract and commercially utilize the lignin recovered from natural ligno-cellulosic materials such as wood. This objective has been highlighted in recent years with the public cognizance of an energy shortage. Climbing prices for oil and natural gas have drawn attention and effort to methods of exploiting the lignin ingredient of wood as a source of plastics feed stock.
Lignin, which is a by product of the pulp and paper industry, is available in large quantities. Because of its complex nature and complicated chemical structure, however, it has not been considered as a valuable chemical intermediate.
Presently, lignin is used almost exclusively as fuel to power the evaporators of the chemical recovery processes and liquor concentration systems of pulpmills. Applicants share the belief of other lignin chemists that lignin can achieve a higher value as industrial raw material than as a fuel.
First, lignin may assume a role as "feed stock" for low molecular weight materials such as phenols which are base chemicals of many products. However, a competitive advantage of lignin over some petroleum or other fossil materials would be best insured by converting it into polymeric materials which retain lignin's structural characteristics. Secondly, polymer modification, rather than breakdown and resynthesis, appears to be another promising approach to the utilization of lignin. Fertilizers, ion exchange resins, and polyurethane products, to name a few, are candidates for such lignin outlets. A third possibility presents itself through a rapidly developing microbiological engineering technology, which views lignin as a natural "protein-precursor". Finally, lignin may also be viewed as a valuable component of high yield pulp.
Lignin is the second most abundant substance in wood, exceeded only by cellulose. It occurs in amounts ranging from 20 to 35% of natural wood content depending on the species, as well as in other parts of the tree such as leaves, shoots, stalks, branches, trunks, and roots. Lignin is thought of as a light brown amorphous "cement" that fills the gaps between the long, thin polysaccharide fibers in the cell walls and binds them together. The role of lignin in gluing the plant fibers together can be compared to that of the polyester resin which is used to strengthen the fiberglass webbing of an automobile body.
Paper producers use various alkaline and/or acidic chemicals to dissolve lignin and to liberate the fibers for papermaking. For them, the lignin is an undesirable wood component.
Presently, there are two main methods in use for removing lignin from wood, the sulfite process and the kraft process. In the sulfite process, the wood is cooked with various salts of sulfurous acid. In the kraft process, wood is cooked with a solution containing sodium hydroxide and sodium sulfide. The dark solutions of the degraded lignin which are dissolved out from the wood are commonly known as "spent sulfite liquor" in the sulfite process, and "black liquor" in the kraft process. These spent pulping liquors are usually concentrated for use as fuel, and for the recovery of certain pulping chemicals.
Lignin may be produced in ways other than the sulfite and kraft processes. These biomass-to-chemicals conversion processes include process schemes based on the involvement of mineral acid (acid hydrolysis lignin, AHL), water and steam at various temperatures and pressures (autohydrolysis and steam explosion lignin, SEL), and organic solvent mixtures, such as ethanol and water (organosolv lignin, OSL). Milled wood lignins (MWL) constitute laboratory preparations isolated by extensive mechanical ball milling of solvent extracted sawdust, and subsequent lignin extraction and purification. MWLs are presumed to be closely representative of native lignins in wood.
The unique chemical and physical properties of the lignin-derived polymer has given it a place among specialty polymer applications such as dispersants, emulsifiers and binders. For these purposes a part of the lignin is recovered from the spent pulping liquors. The reduction in heat value of these liquors is thereby made up with other fuels.
The previously somewhat limited commercial utilization of lignin is occasioned principally by its physical and chemical characteristics. For example, lignin is not resistant to water and is soluble in alkaline solutions. Moreover, lignin is a nonthermosetting thermoplastic which tends to disintegrate if heated above 200.degree. C. and which, if formable at all from the amorphous powdered condition in which it is recovered, merely provides a crumbly mass of little or no strength.
As can easily be extracted from the foregoing one of the goals in lignin chemistry is to develop alternate uses for lignin whereby this unique renewable natural polymer can be disposed of more profitably than presently occurs.
Lignin is composed of carbon, hydrogen and oxygen in different proportions. Its basic building units are phenylpropanes which are interconnected in a variety of ways by carbon-carbon and carbon-oxygen bonds, giving lignin a complicated three-dimensional structure. The molecular weight of lignin varies with its method of isolation, and its source. Lignin from a sulfite pulping process generally has an average molecular weight of about 20-100 thousand. Lignin from kraft pulping processes on the other hand has a lower average molecular weight which ranges from 1.5-5 thousand.
Another characteristic of lignin is that the number of hydroxyl groups (and especially of phenolic hydroxyl groups) per given weight increases as the molecular weights of the lignins decrease. Because low molecular weight lignin possesses a higher percentage of phenolic hydroxyl groups, it has a higher potential to react with oxyalkylating modification reagents such as ethylene oxide, propylene oxide and others. Apart from the reactive hydroxyl sites, lignin possesses various carbonyl, carboxyl, aldehyde and ethylene groups which provide additional sites for other modification reactions.
The chemical pulping agents generally referred to above degrade lignin into a condensed spherical core polymer with reduced activity when compared with that which exists in its naturally occurring state. This is possibly due to the higher surface tension spherical form which may cause the lignin to become a difficult-to-modify material. Notwithstanding this negative factor, lignin has been used in various products because of its availability.
As alluded to above, in general plastics applications, there are two possible ways to utilize lignin. Firstly, lignin may be degraded into low-molecular weight compounds commonly referred to as feed stocks and then reconverted to various synthetic polymers. Secondly, lignin may be used in its natural high-molecular weight state following suitable chemical modification. Such modifications may utilize and act upon any one of the many functional groups present in the complex lignin polymer.
While these general approaches appear simple, they are complicated in both application and intended result.
One method of treating lignin in order to make a potentially useful product involves forming lignin-based epoxides. In the past, such epoxides have been formed by reacting unmodified (phenolic), or phenolated (phenol-enriched) lignin with epichlorohydrin in aqueous alkali. See, e.g., U.S. Pat. Nos. 3,905,926 and 3,984,363 and references to work by Tai et al., Mokuzai Gakkaishi 13 (1967) 257-262. However, the epoxides produced by such reactions possess exceedingly poor solubility, which greatly hinders their commercial utility. Accordingly, it would be useful to develop a method of producing a lignin-based epoxide that exhibits good solubility.
Unmodified (phenolic) lignins have also been treated with methacrylates. See, e.g., Naveau, "Methacrylic Derivatives of Lignin," Cellulose Chemistry and Technology 9: 71-77 (1975). However, poor solubility characteristics of the lignin adducts with methacrylic anhydride or methacrylyl chloride limit their ability for copolymerization with vinyl monomers, and this hinders their commercial utility. Accordingly, it would be useful to develop a method of producing a lignin-based methacrylic acid derivative that exhibits good solubility.
Hydroxyalkylation of lignin has been recognized as a promising technique for overcoming lignin's poor solubility and its frequently observed adverse effects on mechanical properties of solid polymers and on viscosity and cure rate of resin systems. Chemical modification by oxyalkylation has been demonstrated to offer a route to improving solubility, to reducing the brittleness of lignin-derived polymers, and to improving viscoelastic properties in various end uses. Oxyalkylation results in a copolymer combining covalently high modulus lignin with a lower modulus aliphatic polyether phase.
The hydroxyalkylation reaction of various types of lignin and lignin analogs (tannins, etc.) with 1,2-oxides, 1,2-carbonates, and 1,2-sulfites has been accomplished with or without catalyst (alkali metal or alkali earth metal hydroxide or carbonate); at temperatures between 20.degree. C. and 250.degree. C.; in the presence or absence of a solvent; with reactive (alcohols, amines) or unreactive (benzene, chlorobenzene) solvents; and in weight ratios between lignin and oxyalkylating agent between 3:1 and 1:100. See, e.g., U.S. Pat. Nos. 3,476,795 and 3,546,199. The formation of liquid polyhydroxy compounds (polyols) from lignin and carboxylated (by reaction with maleic anhydride) lignin derivatives has been achieved batchwise at temperatures in excess of 180.degree. C. See U.S. Pat. No. 4,017,474. Another approach employs reaction conditions involving incremental addition of alkylene oxides to the heated and stirred reactor content such that the reaction pressure is maintained at about 50 psig. See U.S. Pat. No. 3,546,199. Tarry polyols produced by batch reaction at temperatures above 180.degree. C. have recently been fractionated analytically and preparatively into liquid alkylene oxide homopolymers and solid copolymers with lignin contents of around 60%.